The invention relates to an optical transmitting and receiving device, e.g. as follows from European Patents EP 0 312 788 A2, EP 0 112 498 A2, EP 0 122 609 A1 or German Patent 196 20 147 A1.
In addition to its use for the transmission of digital data, optical transmitting and receiving devices are also used, for example, in analog applications for scanning spaces and structures, as light barriers, for measuring distances or detecting the number of seats occupied in motor vehicles. In the process, light is preferably used in the invisible wavelength range, particularly infrared light. Normally, the transmitting and receiving devices are protected against environmental influences by a housing, which is designed to be permeable to light in the corresponding wavelength range, at least in the area of the transmitting and receiving devices. The problem generally occurs that the light-permeable covers become dirty and the light yield is thus reduced.
On the one hand, the receiving signal levels depend on the distance to the reflecting surface and its remission. Thus, a low or missing receiving signal can be traced back to poor reflection at the surfaces outside of the transmitting and receiving device. On the other hand, it can also point to a defect in the transmitting device. The current-flow control for checking the failure of the transmitting device furthermore is not absolutely reliable.
Solutions that call for an increase in the transmitting output to compensate for the lowering of the receiving level due to dirty conditions must be evaluated especially critically. Thus, a considerable transmitting output is already required for scanning longer distances in the clean condition. If the output is increased further, damage to people in the scanned region cannot be ruled out completely over the long term. This should be avoided, particularly for the use of optical transmitting and receiving devices in the head region of people, as is standard practice for detecting the number of seats occupied.
The German Unexamined Published Patent Application 196 20 147 A1 discloses a device for measuring the line of sight, for which a share of the transmitted light is conducted outside of the light-permeable cover via glass fiber to a receiving device for a comparison evaluation. The soiled cover initially damps the reference beam, generated in this way, in the same way as the actual useful beam. To be sure, in the receiving device the reference beam can be used in a suitable manner for the comparison to the useful beam that is reflected in the observation area, so as to obtain information on the observation area according to the problem definition. However, this only takes into consideration the soiling through a weakening that is identical for both beams, but does not and is not intended to represent in particular a determination of the soiling. A quasi also error-encumbered variable is purposely used for the comparison (see lines 35-39 in column 2).
The European Patent 0 312 788 A2 shows a device for measuring the line of sight by means of a transmitting and receiving unit located inside the motor vehicle cab, where the windshield can be interpreted to be a light-permeable cover that is subjected to soiling. A low share of the light that is diffuse reflected at the windshield travels to the receiving unit, where it can be distinguished from the light shares reflected on the outside of the windshield and can be evaluated owing to the different transit times. The disadvantage in this case is the extremely small share of light that is diffuse reflected at the windshield and actually travels back to the receiving unit. For that reason, the device can provide only a very inaccurate determination of the soiling.
The European Patent 0 112 498 A2 discloses an arrangement for the automatic cleaning of windows, in which a separate transmitting and receiving unit for measuring the degree of soiling is suggested in one embodiment, which is arranged at a different angle to the light-permeable cover. Thus, the arrangement does not affect the measuring result from the other transmitting and receiving unit, but leads to considerable additional expenditure. European Patents 0 112 498 and particularly 0 122 609 disclose another alternative for guiding the light share, reflected at the light-permeable cover of the transmitting unit, by means of light-wave conductors to the receiving unit.
However, a time-window related differentiating between reflections at the inside and at the outside of the light-permeable cover is required in this case, which is involved with respect to control technology. In addition, the expenditure is relatively high, owing to the light-wave conductor, and the share of light reflected at the light-permeable cover that can be detected by means of this light-wave conductor, is very low because of the small opening.
Thus, it is the object of the invention to present an optical transmitting and receiving device, which makes it possible to forego a time-window related differentiation between reflections on the inside and on the outside of the light-permeable cover. The object is furthermore to increase in a simple manner the share of light reflected at the light-permeable cover, which can be detected.
These objects are solved with the characterizing features in patent claims 1 and 2. Advantageous modifications of the invention follow from the features in the dependent claims.
The light shares that are reflected unavoidably on the inside of the light-permeable cover and increase strongly with increased soiling can subsequently be conducted by light-conducting means to a receiving unit. As a result, it is possible to evaluate this internally reflected light share. On the one hand, this light share strongly depends on the degree of soiling while, on the other hand, the arrangement, structure and color conditions outside of the transmitting and receiving device do not affect it.
In order to be able to omit a time-window related differentiation between reflections on the inside and on the outside of the light-permeable cover, light is emitted according to patent claim 1 by a specific transmitting unit, for which the share of light reflected on the light-permeable cover is conducted via the light-conducting means to a first group of receiving units, while a second group of receiving units detects only the share of light reflected on the outside of the light-permeable cover. Thus, a receiving unit can receive only the light reflected on the inside of the light-permeable cover or the reflections occurring on the outside of the light-permeable cover.
A particularly high share of the light reflected on the light-permeable cover can be conducted by means of two parallel reflection surfaces to the receiving unit, wherein these surfaces can be realized particularly easily.
The optical transmitting and receiving device according to the invention can be realized simply, but is nevertheless extremely effective. Owing to the error-free detection of the outside effect of the degree of soiling, the analog receiving level can be corrected on the one hand with a correction factor or, if a threshold value is exceeded, a cleaning of the light-permeable cover can be initiated. Thus, it is conceivable that a cleaning device is actuated or an indicator is actuated, which urges the user to clean the light-permeable cover. There is no danger to the health of persons in the transmitting range. An increase in the transmitting level is not required.
The ratio of diffuse reflected light with total soiling and with no soiling and thus the detection of the degree of soiling can be optimized in that the distance between light-permeable cover and the transmitting and receiving units parallel to the transmitting direction is accordingly adapted to the distance perpendicular to the transmitting direction between the transmitting and receiving units. This distance, in turn, is selected in dependence on the size and distance of the target region.
A numerical ratio of the distances between 4,5 and 6,0 is conceivable in this case, for which an optimum of the-ratio between reflected light with total soiling and no soiling is found.